层序遍历：用一个队列保存当前结点的左右孩子以实现层序遍历，因为先访问的结点，其左右孩子结点也要先访问
1 void LevelOrder(TreeNode* root,vector<int>& res){ 2 if(!root) return; 3 queue<TreeNode*> q; 4 TreeNode* node; 5 q.push(root); 6 while(!q.empty()){ 7 node=q.front(); 8 q.pop(); 9 res.push_back(node->value); 10 if(node->left) q.push(node->left); 11 if(node->right) q.push(node->right); 12 13 } 14 }
接下来可以看两个变种：
Maximum Depth of Binary Tree：
Given a binary tree, find its maximum depth.The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.
1 class Solution { 2 public: 3 int maxDepth(TreeNode* root) { 4 if(root==NULL) return 0; 5 queue<TreeNode*> q; 6 TreeNode* node=NULL; 7 int count=0; 8 q.push(root); 9 while(!q.empty()) 10 { 11 count++; 12 int len=q.size(); 13 for(int i=0;i<len;i++) 14 { 15 node=q.front(); 16 q.pop(); 17 if(node->left) q.push(node->left); 18 if(node->right) q.push(node->right); 19 } 20 } 21 return count; 22 } 23 };
Minimum Depth of Binary Tree：
Given a binary tree, find its minimum depth.
The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.
/** * Definition for a binary tree node. * struct TreeNode { * int val; * TreeNode *left; * TreeNode *right; * TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */ class Solution { public: int minDepth(TreeNode* root) { if(root==NULL) return 0; queue<TreeNode*> q; TreeNode* node=NULL; int count=0; q.push(root); while(!q.empty()) { count++; int len=q.size(); for(int i=0;i<len;i++) { node=q.front(); q.pop(); if(node->left==NULL&&node->right==NULL) return count; if(node->left) q.push(node->left); if(node->right) q.push(node->right); } } // return count; } };